A cast product having an amorphous or glassy atomic structure, in the form of a relatively thin elongated ribbon, has proven to be effective for winding into highly efficient cores for electrical transformers or other uses. Some of the most recent developments in the casting of amorphous or glassy metal ribbons are reviewed in U.S. Pat. No. 4,332,848.
As is known, the casting of ribbons having an amorphous or glassy structure requires the application of a particularly high chill rate to the ribbon on the order of 10.sup.5 -10.sup.6 .degree. C./second Further, this chill rate must be provided throughout the entire thickness of the ribbon if a continuous glassy structure is to be obtained. Consequently, the thickness of amorphous cast ribbons is limited by these extreme heat transfer requirements. If proper heat transfer is not maintained some crystalization occurs, thus destroying the amorphous structure.
The thinness and flexibility of the cast ribbons makes handling of ribbons difficult. Further, when these thin ribbons are stacked in a transformer core, the thinness of the ribbons causes some difficulty with stacking efficiency which in turn causes an apparent density that leaves room for improvement.
Increased stacking or packing densities are possible with thicker ribbons. Advantageously, along with the increased packing density of thicker ribbons there is a characteristic significant increase in the efficiency of the transformer and generally a lower cost. These advantages are deemed to make it worthwhile to seek a way to successfully make thicker amorphous metal ribbons or strips.
U.S. Pat. No. 4,529,458 to Kushnik, et al., assigned to Allied-Signal Corporation, discloses one particularly successful method for making relatively thick composite amorphous strip by compacting relatively thin cast amorphous ribbons. Following compaction, the composite or consolidated strip is subjected to annealing to remove stresses generated during formation and provide improved magnetic properties. As disclosed in the U.S. Pat. No. 4,529,458 patent, a conventional two-stage batch anneal is utilized. In this two-step approach the strip is first annealed at high temperature to relieve stress and then subjected to a standard field anneal.
While this method of annealing is effective in improving the overall magnetic properties of the composite strip, it is not without its disadvantages. First, with this technique the resulting magnetic properties of the composite strip approach, but do not equal, the magnetic properties of an annealed, unconsolidated ribbon. Secondly, this annealing technique is time consuming, relatively expensive and inefficient.
Thus, a need is identified for a new method of annealing composite strip producing improved results. The new method should also be faster, and relatively less expensive by eliminating the need for high temperature furnaces. A method providing distinct advantages when annealing either unconsolidated, as-cast ribbon or composite strip would be very desirable.